Disposing of Backwash Water Correctly: Sewer, Infiltration and Legal Requirements
Backwash water from the sand filter legally counts as wastewater and may not be disposed of the same way everywhere. This article explains the two disposal routes, how to deal with the chlorine content and how to reduce the volume produced.
Backwash Water Is Wastewater, Not Just Pool Water
During every backwash of a sand filter, the filter bed is flushed in reverse and the dislodged dirt is discharged along with the water. Legally, this water counts as wastewater rather than clean pool water: alongside suspended solids, it also contains residues of disinfectants, pH correctors and flocculants. Where it may be discharged is governed by the German Water Resources Act (Wasserhaushaltsgesetz) and, above all, by the bylaws of your municipality.
There are essentially two routes for disposal: discharge into the public sewer or infiltration on your own property. Neither is equally permissible everywhere, and infiltration often requires official approval. Clarify this during the planning phase, because relocating the backwash line afterwards is only possible with considerable effort.
Sewer or Infiltration: The Two Disposal Routes
When discharging into the sewer, a distinction must be made between combined and separate systems. In a separate system, backwash water belongs in the foul water sewer, never in the stormwater sewer, since the latter discharges into a body of water without treatment. In a combined system, everything flows to the sewage treatment plant anyway. The backwash line is usually run in d50 (DN40) or d63 (DN50) and laid with a fall towards the connection point.
| Criterion | Discharge to sewer | Infiltration |
|---|---|---|
| Permissibility | usually possible (foul water sewer) | depends on location and soil |
| Approval | per the municipality’s drainage bylaws | water-law permit often required |
| Chlorine/chemistry | follow the municipality’s requirements | especially critical, chlorine near zero |
| Costs | possible wastewater charge | no ongoing charge, but conditions apply |
Under water law, infiltration is generally regarded as the use of a body of water and therefore often requires a permit from the lower water authority. Chlorinated water is particularly sensitive here. Whether and under what conditions infiltration is allowed is decided by the authority on a case-by-case basis; blanket information from the internet is no substitute for this assessment.
Reducing Chlorine Content and Chemistry Before Discharge
Free chlorine and its by-products place a burden on soil and water bodies. Wherever backwash water infiltrates the ground or enters a separate system, the free chlorine content should be as low as possible, ideally near zero. Binding limits are set by your municipality, so do not rely on second-hand figures but ask the responsible authority. In practice, simple measures help:
- Reduce or suspend dosing one to two days before the planned backwash so that the free chlorine breaks down.
- Preferably backwash before a shock chlorination, not immediately afterwards.
- If necessary, store the water temporarily in a container until the chlorine has dissipated.
Special case, salt chlorinator: salt water pools introduce an elevated chloride load. Salty backwash water is usually unsuitable for infiltration and for watering plants; here, discharge into the foul water sewer is generally the safer route. Confirm this too with your municipality.
How Much Water Is Produced per Backwash?
The volume depends on the filter size, the pump’s flow rate and the flushing time. As a guide: with a flow rate of roughly 8 to 12 m³/h and around two minutes of backwashing plus a brief rinse, roughly 300 to 600 litres are produced per cycle. Over a season with weekly backwashing, this quickly adds up to several cubic metres.
Worked example: 10 m³/h corresponds to about 167 l/min. Two minutes of backwashing yield about 330 litres, a half-minute rinse a further 85 litres or so, making around 415 litres per backwash in total. With 25 backwashes over the season, that amounts to a good 10 m³, which is topped up as fresh water and disposed of as wastewater. Where a wastewater charge is billed based on fresh water consumption, this volume also shows up in the running costs.
Reducing Backwash Water
The less often and the more briefly you backwash, the less wastewater and fresh water are produced. These levers are effective:
- Backwash as needed rather than by the calendar: a backwash is only due once the pressure gauge reads around 0.3 to 0.5 bar above the value for a clean filter.
- Change the filter media: filter glass captures finer particles and extends the intervals compared with quartz sand; filter balls likewise require less rinse water.
- Do not overrun the flushing time: watch the sight glass and end the process as soon as the water runs clear.
- Cartridge filters produce no backwash water at all by design, but are only an alternative for smaller pools.
- Treatment/recycling: for large or public pools, ultrafiltration units treat the backwash water and return it to the circuit.
Municipal Duty to Check: The Questions to Ask the Authority
Because the requirements vary from municipality to municipality, there is no uniform nationwide answer. Depending on the route, the responsible body is either the wastewater utility or the municipal drainage department for the sewer, or the district’s lower water authority for infiltration. It is best to enquire in writing, so that you have the information on record. These sample questions cover the essentials:
- May the backwash water from my private pool be discharged into the foul water sewer, and are wastewater charges payable for it?
- Is infiltration on my property permitted, and do I need a water-law permit for it?
- What requirements for chlorine content and other constituents must the water meet before discharge?
- Do different rules apply to water from a salt chlorinator?
- Are there volume limits or a duty to notify?
- Which authority is specifically responsible, and in what form must proof be provided?
If in doubt, have the connection of the backwash line to the sewer or infiltration system carried out by a specialist company, and have the legal framework confirmed in advance by the responsible authority.
Bill of materials
Example – please verify with a specialist. Parts and quantities depend on the installation; sizes shown are examples.
| Component | Size | Art. No. | KLN No. | Qty |
|---|---|---|---|---|
| Backwash water treatment (ultrafiltration) | – | – | – | Recycling system that treats backwash water and returns it to the circuit; relevant mainly for large or public pools. |
FAQ
May I simply discharge backwash water into the gully or onto the lawn?
Not without further ado. The street gully usually leads into the stormwater sewer and thus untreated into a body of water, which is not permitted. Infiltration on the lawn often requires a permit and is problematic with chlorinated water.
Do I have to register the disposal with the local authority?
Discharge into the sewer is governed by the drainage bylaws, while infiltration often requires a water-law permit from the lower water authority. Enquire in writing with the responsible authority before installation.
How much water do I lose per backwash?
Depending on the pump and flushing time, typically about 300 to 600 litres per cycle. Over a season this can easily add up to several cubic metres that you top up as fresh water.
Does backwash water from salt water pools need to be handled differently?
Yes. The salt chlorinator raises the chloride load, so that infiltration and watering plants are usually ruled out. As a rule, discharge into the foul water sewer is the safer route, agreed with the municipality.
This guide provides general technical orientation and does not replace professional planning. Safety-, electrical- and permit-relevant work must be carried out by qualified specialists.
